How do you prevent collision between two RF modules (ASK modulation) both operating on same frequency (434 Mhz)?

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No you cant.....

Without a encoder like HT12E and decoder HT12D, you cannot differentiate between Tx_A and Tx_B. But with the encoder/decoder pair you can. Study the datasheet of both these chips and ask any questions about them to clear your doubts.

But I use the same RF modules with a AVR MCU and so far I have found nothing that can help differentiate data coming from two different transmitters on same frequency!

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That's because you dint implement the encoder/decoder software onto your AVR chip. If you want that function, you have to put an AVR on the Tx module as well as one AVR on the Rx Module for it to work.

If you are only using the RF module for communication using UART then you need to use different frequencies RF modules to prevent collision. I know there are 2 major type of modules available in the market 315MHz and 433 Mhz.

NO ! I actually meant two separate transmitter and receivers being used with two different sets of MCU.

Let me give a simple example

Say I run a remote controlled car which use the 434MHz ASK RF module with transmitter in remote and receiver in the car.

Now If i have two of those car and two of remote controllers then how would the receiver differentiate which remote is transmitting to it? in other words how can one use two remotes at the same time without them interfering each other?

The receiver would ignore any command unless it came from the right device.

As for "interfering" you should check the spec for that band, if I remember right there is a requirement that each transmission must be less than 10 seconds, and can only transmit about once per minute.

That is how multiple devices can transmit "at the same time" and is common for household gadgets that have multiple remote sensors (multiple transmitters).

There's no point arguing with the messenger, that is simply how it is done.

Simple 434MHz devices just transmit a small packet about once a minute each, and most of their transmissions come through ok.

Yes some packets will collide, and just as likely some packets will be damaged by RF noise or interference.

It does not require a specific protocol unless you require that every packet is received perfectly, which will always require a checksum/re-transmit system, regardless of which band or how many devices.

You said you wanted to know "how do differentiate data coming from two different transmitters" which is a simple beginner question that I have answered.